For example, in an assembling line for automobiles, a part of a coating material used for coating a body is adhered even to nuts and tap holes of accompanying exterior parts (especially, undercarriages, floor panels, etc.) in some cases. Normally, masking is preliminarily applied to nuts and threaded holes such as tap holes provided in members to be coated in order to prevent the coating material from entering there. The operation of attaching/detaching the masking material in the assembling line, however, is complicated. In addition, this operation can be a major factor for deterioration in assembling operation efficiency and increase in cost, especially in the case of members such as large panels having a number of threaded holes to be masked.
Thus, there have conventionally been proposed techniques which, even without masking or coating film removal, enable direct fastening of a bolt to a threaded hole to which a coating film is adhered. For example, Patent Literatures 1 and 2 disclose bolts for peeling a coating film, including notch parts for peeling a coating film, the notch parts being formed over a plurality of pitches at top parts of screw threads in a male screw part formed in a shaft part. These bolts for peeling a coating film can be fastened while peeling a coating film adhered to the internal surface of a threaded hole by means of the notch parts.
In the conventional bolts for peeling a coating film, however, the notch parts for peeling a coating film are commonly arranged in parallel with an axial line of the shaft part as shown in FIG. 1 of Patent Literature 1 and FIG. 1 of Patent Literature 2. Also in the case where the notch parts are arranged to be inclined to the axial line of the shaft part, the inclination angle is small, i.e., around 10°.
One industrial manufacturing method for a bolt is a rolling method including holding a columnar material, which is referred to as a blank, between a pair of dies and rotating the blank while moving these dies in mutually opposite directions for threading. When a bolt for peeling a coating film in which notch parts are arranged in parallel with an axial line is rolled as described above, screw rolling is performed using dies including a plurality of protrusions formed to form notch parts at the same positions. Then, forming resistance greater than that applied to a portion which is merely threaded without any protrusion acts impulsively at the moment when the blank has reached the position of these protrusions. Especially when many protrusions are aligned on the axial line to form a number of notch parts, this tendency becomes strong. As a result of this, outward force acts on the dies at the moment when the notch parts are formed, and thus deep notch parts are disadvantageously difficult to form.
In addition, there is also a method for rolling a screw including preliminarily forming a vertical groove in a blank and applying normal dies to this blank. This method enables forming of deep notch parts. However, since the edges of the vertical groove are crushed at the time of rolling, it is difficult to form notch parts having a sharp sectional shape. Therefore, the bolts for peeling a coating film in which the notch parts are arranged in parallel with the axial line involve the problem that deep and sharp notch parts cannot be rolled. Although deep and sharp notch parts have excellent coating film peeling effect, shallow and non-sharp notch parts cannot sufficiently peel the coating film.
Also, the conventional bolts for peeling a coating film involve the problem that the coating film peeled by the edges of the notch parts remains within the notch parts in a compacted state, and is hard to discharge to the tip end side of the bolts. When the notch parts are clogged by the coating film, the torque resistance at the time of fastening the bolts increases, whereby sufficient fastening force cannot be obtained.